What Is Gestational Age? Definition, Calculation and How It Differs from Fetal Age
Source: NHS, American College of Obstetricians and Gynecologists (ACOG) | Last reviewed: April 2026
Gestational age is the standard way of measuring how far along a pregnancy is. It is used by all healthcare providers — midwives, obstetricians, sonographers, and neonatologists — to schedule antenatal care, assess fetal development, and determine when a baby is at term.
Definition
Gestational age is the age of a pregnancy counted from the first day of the last menstrual period (LMP), expressed in weeks and days.
This is the number your healthcare provider refers to when they say "you are 12 weeks pregnant" or "the baby was born at 34 weeks." It is not the same as how old the embryo or fetus is from the moment of fertilisation.
Gestational Age vs. Fetal Age
| Gestational Age | Fetal (Embryonic) Age | |
|---|---|---|
| Counted from | First day of LMP | Date of conception/fertilisation |
| Difference | ~2 weeks longer | ~2 weeks shorter |
| Full term | 40 weeks | ~38 weeks |
| Used by | Healthcare providers (standard) | Embryology, IVF settings |
Because ovulation (and therefore conception) typically occurs around day 14 of a 28-day cycle, gestational age is approximately 2 weeks more than fetal age.
Example: A fetus at 38 weeks of fetal development has a gestational age of approximately 40 weeks.
Why the LMP Is Used as the Starting Point
The LMP is used as the start date because:
- It is a known, recordable date the pregnant person can usually recall
- The exact date of conception is almost always uncertain without specialised monitoring
- It creates a consistent, standardised reference point used globally in obstetric practice
How Gestational Age Is Calculated
Method 1: From LMP (Naegele's Rule)
LMP + 280 days = estimated due date (40 weeks gestational age)
Example: LMP of 1 January 2026 → due date approximately 8 October 2026
Method 2: Ultrasound
An ultrasound between 8 and 14 weeks measures the crown-rump length (CRL) — the distance from the top of the head to the base of the spine. This is compared to reference tables to estimate gestational age.
First trimester ultrasound dating is accurate to within 5–7 days and is the gold standard when LMP is unknown or uncertain.
Gestational Age Classifications
| Classification | Gestational Age |
|---|---|
| Extremely preterm | Before 28 weeks |
| Very preterm | 28–31+6 weeks |
| Moderate to late preterm | 32–36+6 weeks |
| Early term | 37–38+6 weeks |
| Full term | 39–40+6 weeks |
| Late term | 41–41+6 weeks |
| Post-term | 42 weeks and beyond |
A baby born before 37 weeks is preterm. Most guidelines recommend delivery monitoring from 41 weeks, with induction typically offered by 42 weeks to reduce post-term risks.
How Gestational Age Structures Antenatal Care
| Gestational Age | Standard Check or Scan |
|---|---|
| 8–14 weeks | Dating scan (confirms gestational age) |
| 11–14 weeks | Combined first trimester screening |
| ~16 weeks | Midwife appointment, blood results review |
| 18–21 weeks | Anomaly (mid-pregnancy) scan |
| 24–36 weeks | Regular appointments every 2–4 weeks |
| 38–40 weeks | Final check-ups |
| 41–42 weeks | Post-dates monitoring and induction discussion |
Related Guides
- How to Calculate Your Due Date — LMP, conception, and ultrasound dating methods
- LMP vs Conception Date — when the two methods give different answers
- Pregnancy Trimesters Explained — key milestones at each stage
- Pregnancy Due Date Calculator — calculate your EDD instantly
This guide is for general information only, based on NHS and ACOG guidance. Gestational age confirmed by ultrasound takes precedence over LMP-based estimates.